The inventive concept relates to optical devices and, more particularly, to single-mode distributed feedback-laser diodes.
A distributed feedback-laser diode (DFB-LD) has been applied to various kinds of high speed optical communication systems because of excellent oscillation properties and modulation properties thereof. The modulation properties of a device may be influenced by resonance properties and parasitic effect of a laser diode. And single mode properties may be closely related to a device structure such as a reflective index of an output facet, a shape of a grating, and/or a coupling coefficient.
The DFB-LD may include an active layer and a grating disposed on or under the active layer. A refractive index or a loss may be periodically repeated on the grating. A laser light may be oscillated with a wavelength satisfying Bragg condition in a effective refractive index change structure (i.e., index-coupled grating) of a waveguide in the DFB-LD. The grating may oscillate the laser light by satisfying a phase-condition at two wavelengths within a stop band. The grating may oscillate two wavelengths in a spectrum of the laser light. Kinds of conventional DFB-LDs having the gratings will be described below.
Firstly, a conventional DFB-LD may have a high reflection (HR) coating and an anti-reflection (AR) coating which are cross-sections of the DFB-LD, respectively. The wavelength of the laser light may be very freely changed depending on the coupling coefficient of the grating, a reflective index of the HR coating, and/or an operation current, so that the conventional DFB-LD may have limited oscillation properties. Thus, since a phase of the grating may be randomly shown in the HR coating facet of the conventional DFB-LD, a yield of the single mode may be about 20% or less.
Secondly, another conventional DFB-LD may include a phase-shift grating which is shifted from a predetermined position in a cavity by a quarter of a wavelength. However, the phase-shift grating may be formed by a high cost apparatus such as an electron-beam lithography apparatus. Thus, productivity may decrease.
Thirdly, still another conventional DFB-LD may include a gain/complex-coupled grating where optical loss or optical gain is changed in a longitudinal direction of the index-coupled grating. However, since an oscillated wavelength is changed by change of the optical loss or the optical gain of a device in the conventional complex-coupled DFB-LD, it is difficult to control an operation wavelength.